The present invention relates to composite structures for use at high temperature. More particularly, it relates to composites which are formed of materials having relatively lower density and yet which are able to exhibit improved Young's modulus as well as high strength properties at high temperatures.
There is increasing interest in substances and structures which have the capacity for displaying good stiffness at lower temperatures as well as high strength and other compatible physical properties at high temperatures. Where such structures are to be used as part of jet engines, there is also a premium attached to the low density or low weight of the structure. Potential lighter weight, high strength materials and structures which retain their strength at high temperature are difficult to identify and harder still to formulate or to construct. Such structures are nevertheless highly valuable and, because of their high value as components for jet engines, the cost of the materials and articles is considered secondary to the properties which they exhibit at the use temperatures.
Titanium aluminide, Ti.sub.3 Al, as well as other titanium base alloys, have been identified as potentially high strength at high temperature materials having favorable strength-to-weight ratios. Silicon-carbide filaments have been recognized as having very high longitudinal strength features and it has been proposed to form a desirable structure in which the silicon-carbide filaments serve as a reinforcement for titanium aluminide metal and other titanium base alloy bodies. It is anticipated that Ti.sub.3 Al matrix composites will find application in wound rotors, casings, and other intermediate temperature, high stress applications.
At present, Ti.sub.3 Al composites have been fabricated by rolling Ti.sub.3 Al ingot to sheet of about 0.010 inch thickness and laying up alternate layers of Ti.sub.3 Al sheet and arrays of SiC filaments or fibers to form a laminate. The laminate formed in this manner is then consolidated by hot pressing or hot isostatic pressing, i.e. HIPing. This prior art process is deemed to be inadequate and too expensive for use as a high production rate manufacturing process for formation of such composites.
Novel and unique structures are formed pursuant to the present invention by plasma spray deposit of titanium base alloys and titanium-aluminum intermetallic compounds employing RF plasma spray apparatus.
The formation of plasma spray deposits of titanium and of alloys and intermetallic compounds of titanium present a set of processing problems which are unlike those of most other high temperature high strength materials such as the conventional superalloys. A superalloy such as a nickel base, cobalt base or iron base superalloy can be subdivided to relatively small size particles of +400 mesh (about 37 .mu.m) or smaller without causing the powder to accumulate a significant surface deposit of oxygen. A nickel base superalloy in powder form having particle size of less than -400 mesh will typically have from about 200 to about 400 parts per million of oxygen. A powdered titanium alloy of similar particle size by contrast will typically have a ten fold higher concentration of oxygen. A powdered titanium alloy of -400 mesh will have between about 2000 and 4000 ppm of oxygen.
Moreover, titanium alloy powder of less than -400 mesh size is recognized as being potentially pyrophoric and as requiring special handling to avoid pyrophoric behavior.
It is also recognized that the low temperature ductility of titanium alloys decreases as the concentration of oxygen and of nitrogen which they contain increases. It is accordingly important to keep the oxygen and nitrogen content of titanium base alloys at a minimum.
Prior art plasma spray technology is based primarily on use of direct current plasma guns. It has been recognized that most as deposited plasma spray deposits of the superalloys such as nickel, cobalt and iron base superalloys have had relatively low ductility and that such as sprayed deposits when in their sheet form can be cracked when bent through a sufficiently acute angle due to the low ductility.
I have discovered that RF plasma apparatus is capable of spraying powder of much larger particle size than the conventional DC plasma apparatus. I have discovered that particle sizes at least three times larger in diameter than those conventionally employed in DC plasma spray apparatus may be successfully employed as plasma spray particles and that the particle size may be as high as 100 .mu.m to 250 .mu.m and larger and as large as 10.times. as large as the -400 mesh powder previously employed in DC plasma spray practice.
This possibility of employing the larger powder particles is quite important for metal powders such as titanium which are subject to reaction and absorption of gases such as nitrogen and oxygen on their surfaces. One reason is that the surface area of particles relative to their mass decreases inversely to their diameters. Accordingly, a three fold increase in particle diameter translates into a 3 fold decrease in particle surface area. I have discovered that one result is that RF plasma spray deposited structures of titanium base alloys can be made with the aid of larger particles and that they accordingly have lower oxygen content than might be expected based on knowledge of prior art practices.
As used herein, the term titanium base alloy means an alloy composition in which titanium is at least half of the composition in parts by weight when the various alloy constituents are specified, in parts by weight, as for example in percentage by weight.
A titanium-aluminum intermetallic compound is a titanium base alloy composition in which titanium and aluminum are present in a simple numerical atomic ratio and the titanium and aluminum are distributed in the composition in a crystal form which corresponds to the simple numerical ratio such as 3:1 for Ti.sub.3 Al; 1:1 for TiAl and 1:3 for TiAl.sub.3.